Photographic silver halide materials are normally coated on a transparent film base so that, after processing, light may be passed through the negative image to form a positive image on print paper. Problems in coating the film evenly and uniformly with silver halide materials are regularly encountered. The coating must be even or the material will vary in light sensitivity and show coating marks in the final product. It must also be sufficiently well attached to withstand liquid processing, washing and drying, and yet allow the free entry and chemical action of the developer during processing. To meet these requirements natural gelatin is conventionally used in admixture with the silver halides to form a light sensitive emulsion.
Photographic emulsions are prepared by mixing together two solutions, one containing the dissolved silver ions and the other containing the halide ions. Natural gelatin is mixed with these solutions in order to prevent coalescense of the precipitated silver halide crystallites by adsorbing to the newly formed grains at the time of precipitation and maintaining them in suspension. Natural gelatin is particularly effective in these emulsions because it prevents coalescence yet does not prevent or modify grain growth. Natural gelatin so successfully fulfills the rigid requirements for a photographic emulsion that it is today, more than a century after its first use in photography, the most common emulsion material used in photographic manufacturing.
One of the problems encountered in the use of natural gelatins is the batch-to-batch variation in the properties of identically processed gelatin obtained from different sources or from the same source at different times. These differences give rise to non-uniform photographic products which in turn present significant problems to the photographic processing industry.
A great deal of time and effort has been expended in the search for other suitable binders to overcome the problems inherent in the use of natural gelatin. Attempts to replace natural gelatin with synthetic polymers have so far met with only limited success.
Various researchers have studied the interaction of silver ions with natural gelatin (Carroll and Hubbard, J. Res. Nat. Bur. Std., 7:811 (1931); Lanza and Mazza, J. Electroanalyt. Chem., 12:320 (1966)). All of these authors observed binding of silver by gelating at low pH. This effect has been recently attributed to the presence of methionine groups in the protein. See, for example: Russell, J. Photogr. Sci., 15:151 (1967).